The macula is the region of the retina which is used for high acuity vision, as is typically required for reading. To diagnose macular damage, a patient may undergo various types of examinations, including automated perimetry or campimetry, in which the patient is positioned in front of a test surface and is asked to maintain focus on a target. A computer then actuates one or more light sources to present visual stimuli at specific points on the test surface. The patient is asked to press a button in response to perceived test stimuli and the examiner or computer records the patient input and associated spatial information. In this way a visual field map is created.
The human retina is unable to fixate continuously upon an unmoving, unchanging stimulus. After less than a second of stimulation, the subject's retinal cells will adapt to the stimulus and no longer relay any information to the brain regarding the stimulus. Such adaptation is problematic for devices requiring constant fixation upon a fixed point since the viewer must change fixation in order to continue seeing the object of regard. This changing fixation may reduce the accuracy, precision, and overall effectiveness of testing, therapeutic, or non-therapeutic visual stimulation programs that require continuous unchanging visual fixation.
The human visual system includes two simultaneously functioning pathways. The more primitive “M” pathway is dedicated largely to the recognition of moving objects or objects that change in luminosity. The “P” pathway is more evolved and more closely coupled to neural structures associated with conscious thought. The “P” pathway provides recognition of fine detail and color to the brain and is much less committed to motion detection.
In persons who have suffered damage to the visual system (which includes the brain itself), these two pathways frequently are affected to different degrees. Moreover, the two systems usually recover function at different rates. This disparity of functional loss and recovery can cause significant disharmony of the innately matched systems and resulting disturbances to the overall sensory function of the subject.
The ability to accurately detect the different types of visual processing disturbance and to properly treat the affected pathways is of vital importance to the treating practitioner and ultimately, the patient. Increased diagnostic testing sensitivity and visual therapy specificity result in better prognosis for recovery of the individual's function. Current diagnostic testing modalities are limited in their ability to detect specific types of visual function damage.
Most field-of-vision testing falls into one of two categories: Static perimetry and kinetic perimetry. Static perimetry is valuable in establishing a depiction of fine light sensitivity or detail within the central 30 to 60 degrees of visual field (“P” cell function), while kinetic perimetry is valuable in identifying the borders of motion vision function (“M” cell function). However, neither technique is able to identify efficiently the amount or level of motion sensitivity at all the points within the borders.
In static perimetry, the patient fixates upon a specific point while light stimuli (spots) are delivered to various points in the peripheral visual field. Static perimetry can be administered using peripheral stimuli of varying brightness to determine the level of luminance sensitivity throughout the field, or with peripheral stimuli of identical brightness, to screen for the presence or absence of vision at various locations.
Kinetic perimetry also requires a patient to fixate a central spot during delivery of peripheral stimuli. In kinetic perimetry, the stimuli are luminous spots of varying size and brightness. The spots are moved from an area where there is known to be no vision (e.g. the far periphery or physiologic blind spot) toward areas where vision may exist. The patient is tasked with responding upon detection of a moving light in the periphery. Test points are delivered along radii of the circle (or “horopter”) of the patient's field of vision. The points of first detection are recorded on a plot and the circumference of the connected points is considered the border of motion and light sensitivity for the brightness and size of the stimulus used.
Frequency doubling technology (FDT) is used to test the spatial resolution of a subject's visual field. In FDT, a subject attempts to observe a peripheral square or circular grating comprised of striations of alternating luminosities. The gratings are modulated in a wave pattern involving temporal changes in luminosity of the striations. The luminosity modulation is performed above the critical flicker frequency (CFF), typically about 25 Hz, so that the modulation is not noticeable to the subject. If the subject's visual field is sufficient, the subject will observe an optical illusion in which the spatial frequency of the striations is doubled, i.e., the space between the striations is halved. The spatial frequency or contrast may be modulated to determine the limits of the subject's spatial resolution. In a patient with visual field damage, e.g., one suffering from glaucoma, an abnormally low spatial frequency may be required for the subject to observe the frequency doubling illusion. In U.S. Pat. No. 6,068,377, issued May 30, 2000 to McKinnon, an alternate version of FDT is employed, in which the grating is isoluminous, but with alternating hue.
Work in the laboratory of Krystel R. Huxlin at the University of Rochester has utilized a series of spot stimuli for visual field therapy. In the poster presentation, “Training-induced perceptual recovery after visual cortical stroke” Eric Kelts, Jennifer M. Williams, Brad Feldman, Mary Hayhoe and Krystel R. Huxlin, 30th Annual NANOS meeting, Orlando, Fla., 2004, stimuli move with respect to the entire visual field. Such an approach is not consistent with visual restoration therapy approaches that individually target small regions of the visual field. Additionally, movement of a stimulus across a large region will tend to cause distraction of a patient, and is thus inconsistent with visual therapy approaches that utilize a fixation stimulus. If patient feedback were collected with such a system, it would be difficult or even impossible to precisely locate the visual field region that caused the patient's perception to be triggered.